Top contact VCSEL with monitor

ABSTRACT

A vertical cavity surface emitting laser (VCSEL) and monitoring diode combination having reduced parasitic capacitance for use in high bandwidth communications systems. The VCSEL has both p-type and n-type contacts on the same face. This allows the VCSEL to be mounted on a monitor chip or diode without using a metal contact layer. In an embodiment wherein the VCSEL is soldered to the monitor chip for mechanical stability only a small metal pad no larger than the VCSEL is used. The reduction in metallization results in a lower parasitic capacitance that in turn results in higher potential operational speeds

This application is a continuation-in-part of U.S. application Ser. No.09/419,810 filed Oct. 18, 1999.

FIELD OF THE INVENTION

This invention relates to a vertical cavity, surface emitting laser(VCSEL) with associated output monitoring device and more particularlyto such a combination for use in high bandwidth applications.

BACKGROUND OF THE INVENTION

Lasers, and in particular semiconductor lasers, such as those formed ofIII-V compounds, are commonly employed as the transmitter in digitalcommunication systems. Lasers of this type are particularly well suitedfor optical fiber based systems wherein the optical output of the laseris modulated in accordance with an electrical input to the device. Themodulated optical signal can be transferred over relatively longdistances using current optical fiber transmission systems.

Early lasers used in such applications included edge emitting laserswherein the cleaved edges of the device provided the reflecting faces ofthe Fabry-Perrot cavity. Edge emitting lasers, however, have certainlimitations with respect to pre-assembly testing and mounting techniquesfor efficient coupling to small diameter optical fibers.

Surface emitting lasers and in particular vertical cavity surfaceemitting lasers (VCSELs) have been developed recently and provide animprovement over edge emitting devices for use in optical fibercommunication applications. The VCSEL which has its active regionlocated between two reflecting layers, such as Bragg mirrors, emits in aplane normal to one of the two major surface of the laser device. As isknown such devices include material of a first conductivity type, forexample n-type, for one of the Bragg mirrors and material of a secondconductivity type, namely p-type, for the other Bragg mirror. Theintermediate active region or layer may include a cladding layeradjacent each of the Bragg mirrors. The Bragg mirrors arc typicallyformed of alternate layers of III-V semiconductor material each layerhaving a different reflectivity characteristic. Each alternate layertypically has a thickness equal to one quarter wavelength based on theemission wavelength of the active material.

In this structure, and in particular top emitting VCSELs, the back orbottom surface of the device is usually attached to a mounting substrateand the laser output is emitted through the top or front face of thedevice. An emitting aperture, which may be defined by one of the devicecontacts, is typically configured to allow alignment with an opticalfiber. Such devices are fabricated using well established processingtechniques and provide reliable lasers which may be convenientlyassembled into optical transmitter units.

The nature of VCSELs, and indeed semiconductor lasers in general, isthat the electrical and optical characteristics between each device mayvary slightly. The optical output taken as a function of input currentduring lasing action represents a steep slope and minor variations inthe operating environment can result in significant output changes. Forthis reason, it is common to include a monitoring diode or monitor chipwith a VCSEL, wherein the monitor chip is arranged to receive arepresentative portion of the optical output. This representative outputcan be used to calibrate each laser device or it can be used in afeedback mode to control the optical output of the laser. Control may berequired to ensure that the optical output of the laser falls within apreset limit such as might be required by “eye safe” regulationsprescribed by Standards Agencies. Typically the monitoring diode will bea phototransistor such as a PIN device having a sensitivity curvegenerally matched to the wavelength output of the laser.

Laser/monitor combinations are frequently mounted in a speciallydesigned package such as a TO-46 can which has a mounting base withinsulated connector leads and a sealed cover. The cover has a window ofglass or other suitable transparent material over a central portion ofthe top such that the window is aligned with the emitting aperture ofthe lasing device. One such combination is described in U.S. Pat. No.5,812,582 which issued Sep. 22, 1998 to Gilliland et al. In the U.S.Pat. No. 5,812,582 the photodiode is mounted on an insulated substratethat is positioned within in a TO-46 can, or the like. A large portionof the top surface of the photodiode is covered with a metal layer ormask. A VCSEL is electrically attached to the mask by solder orconductive epoxy and one of the contacts to the VCSEL, i.e. the backcontact, is made by way of the mask. The top or emitting surface contactis through a wire bond to one of the isolated connectors in the TO-46can.

The bandwidth capacity of current optical fibers far exceeds thebandwidth utilized by present day communication systems. Accordingly,there is a continuing effort to increase the data rate of communicationsystems in order to make better usage of optical fiber capabilities.Since the laser transmitter represents an important aspect of thecomplete communication system it is important that the switching rate ofthe laser be as high as possible. One factor which effects the switchingrate in high speed devices is the parasitic capacitance of the VCSEL,the monitoring chip and the mounting configuration.

A further important consideration, of Course is the cost of the opticaltransmitter or laser/monitor assembly. This cost includes the materialprocessing costs as well as the cost of assembling the devices and inaccurately aligning the device in relation to an optical fiber.

It is, accordingly, an object of the present invention to provide a lowcost VCSEL/monitor device with reduced parasitic capacitance for highbandwidth applications.

SUMMARY OF THE INVENTION

In a preferred embodiment of the present invention there is provided aVCSEL/monitor assembly in which the VCSEL has both p-type and in-typecontacts on the top or emitting face and is mounted on a monitoringdiode utilizing little or no metallization in the mounting process.

Therefore, in accordance with a first aspect of the present inventionthere is provided a vertical cavity surface emitting laser (VCSEL) andphoto detecting monitor assembly comprising: a photo detecting monitorchip having a first photo detecting face and a second face parallelthereto; a top emitting VCSEL, mounted on the first face of the monitorchip, the VCSEL having both p-type and n-type contacts on a top surfacethereof; and means associated with the assembly to direct a portion ofthe VCSEL emission to the first face of the monitor chip.

In accordance with a second aspect of the present invention there isprovided a method of assembling a top emitting vertical cavity surfaceemitting laser (VCSEL) and photo detecting optical output monitoringchip pair comprising: providing a monitoring chip having a photodetecting surface; attaching a top emitting VCSEL to the detectingsurface, the VCSEL having p-type and n-type contacts on the top surface;and providing means to contact the monitoring chip and the VCSEL.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail with reference tothe attached drawings wherein:

FIG. 1 is a top view of a monitor chip;

FIG. 2 is a top view of a top emitting VCSEL having both contacts on theemitting face;

FIG. 3 is a cross-sectional view of a VCSEL mounted on a monitor chipaccording to the present invention;

FIG. 4 is a cross-sectional view of the assembly in a mounting caseincluding a receptacle or sleeve for use in the connection to an opticalfiber; and

FIG. 5 is a bandwidth curve showing modulation response as a function offrequency.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 represents one example of a monitor chip 10 for use in thepresent invention. The monitor is a photodiode, PIN, avalanche diodeetc. In operation, surface 12 is impinged with optical energy of anappropriate wavelength and this will result in an electrical outputacross contacts on the device. The electrical output will be dependenton the intensity of the optical energy (from the laser in thisapplication) which is received by the photo detecting or monitor chip.One electrical contact is provided on the back surface (not shown) andit may be connected to a lead wire or mounted directly onto a substrateby well known means. Similarly, a contact to the top or front face canbe made by way of wire bonds to one or both of contact pads 14.Alternatively, the monitor diode may have both contacts on the same sideand the electrical connections made by bonding to the pins, for contactson top side, or by means of patterned electrodes on the photodiodecarrier for contacts on the bottom side.

As shown in FIG. 1, centrally positioned on the top surface of themonitor chip is an alignment mark 16. This may be a square, orsubstantial square, as shown in FIG. 1 and correspond to the shape oroutline of a VCSEL, to be described later. Alternatively the alignmentmark 16 may represent a small portion of the VCSEL outline. Thealignment mark can be formed of an appropriate metal in order to allowthe VCSEL to be soldered in an accurately aligned relationship withrespect to the photodiode. This alignment is important in relation tosystem packaging as will be described later. Alternatively, thealignment mark may be of a non-metallic layer as long as it provides astable surface while providing a suitable alignment mark. It is, ofcourse, possible in certain assembly techniques to position the VCSELaccurately on the monitor chip without the aid of a special alignmentmark. For example, the contact pattern or chip edge can be utilized foralignment purposes.

As shown in FIG. 2, the VCSEL 20 according to the present invention hasboth p and n-type contacts on the top surface. An emitting aperture 22will typically be defined by one of the contacts 24 (for example p-type)which will be connected to the p-type Bragg mirror of the VCSELstructure. A second contact 26, n-type in this example, will beconnected to the n-type Bragg mirror as is known in the art. A currentaperture is typically provided in the active region (not shown) in orderto confine the injection current to the desired region of the structure.Contact pads 24 and 26 are formed of suitable material for acceptingwire bonds or the like for connection to a suitable input source.

VCSEL 20 is shown in FIG. 2 as being substantially square while it is tobe understood that other shapes and sizes of devices can be used in thepresent invention. The alignment mark 16 on the monitor chip will, ofcourse, need to be modified to correspond generally to the shape andsize of the VCSEL chip if other configurations are used.

It is also within the scope of the present invention to use a bottomemitting VCSEL having both p-type and n-type contacts on the bottomface.

FIG. 3 shows a cross section of a VCSEL 20 stacked or mounted on themonitor chip 10. If the alignment mark is formed of metal the back faceof the VCSEL chip will have a suitable metallization to allow the VCSELto be attached to the monitor chip by soldering. In this case the VCSELwill be self-aligned to the alignment mark by the soldering process. Itis to be understood that the alignment mark can be restricted to someportion of the size of the VCSEL and still result in a good mechanicalconnection and be suitably aligned.

The VCSEL can also be attached to the monitor chip by a suitableadhesive, many of which are available for such purposes. The alignmentmark in this case will preferably not be metal but some other materialsuch as a dielectric again having a pattern designed to assist anoperator in the proper location of the VCSEL in relation to the monitorchip. As previously discussed certain assembly techniques will allow anoperator to position the VCSEL on the monitoring chip without the use ofany special alignment marks.

Adhesive materials which have proven to be suitable for attachment ofthe VCSEL to the monitor chip include certain epoxies and in particularthermoplastics. One such material is Alpha Metals Staystik Thermoplasticunfilled paste (101, 181) with appropriate thinner. Although the pastecan be applied to the back face of individual die benefits are achievedby applying a layer of the paste to the back face of a fully processedwafer of VCSELs and spreading the paste into a thin uniform layer. Thepaste can be applied by several methods including screening but aparticularly suitable method involves spinning. In this method the pasteis applied to the center of the back face of a fully processed wafer andthe wafer is placed on a spinner such as the type used for theapplication of photoresist. The spinning action causes the paste to beevenly spread over the face by centrifugal force. After spinning thethin layer of paste is dried by heating and thereby fused to the wafer.The temperature profile for the drying cycle is dependent on the pasteused but may be up to 350 degrees C.

The dried and fused paste can be patterned by selectively removingmaterial utilizing conventional lithographic processes. The wafer canthen be scribed along the patterned lines and subsequently broken intoindividual VCSEL devices. Otherwise, the paste layer is left in acontinuous covering and the wafer is placed on the typical blue stickytape commonly used in the industry and the wafer is cut into individualVCSELs utilizing a wafer saw. The individual VCSELs are picked off ofthe blue tape and placed on top of the monitor chip using theaforementioned alignment marks if appropriate.

The combination is then heated to the point where the adhesive pastemelts and upon subsequent cooling the VCSEL is firmly attached to themonitor chip.

The application of a thin, even layer of the paste by spinning, forexample, results in a controllable thickness of the adhesive. The thinlayer results in a much lower incidence of tilting of the VCSEL on themonitor chip when the paste is melted and subsequently cured. This is animportant consideration since the VCSEL is used in an optical system andany misalignment can result in lower coupling characteristics. Further,an excess thickness of the paste will result in reduced thermal transferthrough the device and this can impact device reliability.

The above mentioned technique of applying an adhesive paste to theVCSELs means that when the VCSEL is secured to the monitor chip the gluedoes not spread beyond the laser onto the surrounding monitoring area.This improves the tracking ratio, i.e. how well the monitor currentdescribes the actual output of the VCSEL. Additionally, there isn't anexcess of the adhesive that could contaminate the assembly tools and theVCSEL itself. As an additional benefit the glue protects the fragileback of the VCSEL while the device is being removed from the blue tapewhich may involve pushing it up with a pin or the like.

FIG. 4 is a cross sectional view of the assembly of FIG. 3 mounted in apackage such as a conventional TO-46 can. As shown the can includesposts or connectors 32 which are normally isolated but one may benon-isolated for use in providing electrical input to the VCSEL and formonitoring the optical output of the VCSEL by way of the electricalsignal generated across the monitor chip. The cover 42 includes areflective surface 44 which may be a window or lens. The material forthe reflective surface may be glass, plastic epoxy or other material atleast partially transparent to the wavelength of the VCSEL. A portion ofthe laser output will be reflected by the window 44 back inside thecover and will impinge on monitor chip surface 12 thus providing asignal which is proportional to the VCSEL output.

As will be apparent the positioning of the emitting aperture of theVCSEL with respect to an optical fiber coupled to the arrangement iscritical. Positioning the sub assembly, which may be a TO-46 can, aTO-56 can, a MT connector or other sub assembly, in relation to thefiber can be fixed by way of a receptacle or sleeve. It is thereforeimportant that the VCSEL and hence the emitting aperture is wellpositioned with respect to the base of the TO-46 can. The alignment markon the monitor chip assists in the positioning of the VCSEL with respectto the monitor chip and the positioning of the monitor chip with respectto the package can be arranged through other means. As shown in FIG. 4the central axis of the emitting aperture is aligned with the center ofwindow 44 and the longitudinal axis of an optical fiber (not shown).

By reducing the amount of metal in the VCSEL/monitor assembly of thepresent invention the parasitic capacitance is correspondingly reduced.The prior art device as described in U.S. Pat. No. 5,812,582 utilizes ametal mask or layer on top of the monitor chip. This mask in combinationwith the substrate on which the chip is mounted act as a plate capacitorand the parasitic capacitance produced by it can introduce a delay whichaffects high speed operation. Similarly, the VCSEL contacts, being onopposed faces, create another plate capacitor which also adds to theparasitic capacitance of the combination. In the present invention bothVCSEL contacts are on the top or emitting face thereby reducingparasitic capacitance. Additionally, the metal mask on the monitor chipof the prior art device is not required by the present invention sinceno back face electrical connection is made.

The graphs in FIG. 5 compare the frequency response of devices made bythe present invention (curve A) with devices made by prior arttechniques (curves B and C). As shown the output of devices made inaccordance with the present invention the remains substantially constantup to 1.7 GHz, even when the package is a TO-46 can which is notoptimized for high frequencies. Applications for devices of the presentinvention include low and high speed data communications, for example100 Mbps Ethernet, Gigabit Ethernet, Fiber Channel, and ATM or SDH andIEEE. Applications also include non-fiber applications like medical andchemical where the concentration of a substance is measured by means ofits interaction with photons at the emitted wavelength. In fact, theinvention pertains to any application where a monitor diode is neededfor the feedback of optical power.

While particular embodiments of the invention have been described andillustrated it will be apparent to one skilled in the art that numerousvariations can be effected without departing from the basic concept ofthe present invention. For example the shape and size of the VCSEL andmonitor diode can be selected according to the application. Theconductivity type of the VCSEL and monitor diode is not affected by themounting technique, i.e. a VCSEL having a n-type substrate can bemounted on a p-type, an n-type or an isolating part of the monitor chip.The same is true of a VCSEL grown on a p-type substrate. Additionally,it is contemplated by the present invention to use thin adhesivepreforms instead of the thin adhesive layer applied to the back face ofthe wafer by spinning. It is to be understood, however, that suchvariations will fall within the full scope of the invention as definedby the appended claims.

What is claimed is:
 1. A method of assembling a top emitting verticalcavity surface emitting laser (VCSEL) and a photo detecting opticaloutput monitoring chip pair, comprising providing a monitoring chiphaving electrical contacts and a photo detecting surface; attaching saidVCSEL to said photo detecting surface so that at least a portion of saidphoto detecting surface is uncovered, said VCSEL having a back facewhich is attached to said photo detecting surface with an electricallyisolating adhesive and electrical contacts on a top face; and providingelectrical connections to said monitoring chip and to said contacts onsaid VCSEL.
 2. The method of claim 1 wherein said adhesive is anunfilled thermoplastic paste.
 3. The method of claim 2 wherein saidunfilled thermoplastic paste is applied to said VCSEL while said VCSELis part of a wafer of VCSEL material and thereafter converted to a thineven layer by spinning.
 4. The method of claim 3 wherein an individualVCSEL having a coating of thermoplastic adhesive on said back face ispositioned on said monitor chip and heated to melt the adhesive so thatupon cooling said VCSEL is retained on said monitor chip.
 5. A method ofapplying adhesive to the mounting face of a vertical cavity surfaceemitting laser (VCSEL) for use in attaching the VCSEL to an opticalmonitoring device comprising: applying an adhesive in a viscous form tothe mounting face of a wafer, said wafer including a plurality ofVCSELs, said adhesive being electrically isolating; spinning said waferto spread said adhesive into a thin uniform layer; heating said wafer tocure said adhesive; and separating said wafer into individual VCSELs. 6.The method of claim 5, wherein an individual VCSEL is mounted on amonitor chip and heated to melt said adhesive such that said VCSEL isattached to said monitor chip upon cooling.
 7. The method of claim 5,wherein said adhesive is an unfilled thermoplastic paste.